TDOA is a measurement of the difference in arrival time of two signals originating simultaneously (synchronously in near absolute time) from two different known positions to a common unknown position in space or is the measurement in difference of arrival time of a common single signal of unknown origin to two different known locations in space. This information is valuable in the field of location positioning when the signal velocities are known and/or predictable and are constant and are used to determine the distances between the three objects mentioned above.
TDOA can make use of wired or wireless signals. Most commonly, these consist of light, radio, electrical voltage differentials, or sound transported via waves, particles, pulses and packets, or any combination thereof. TDOA is particularly useful to multilateration algorithms, where a series of TDOA values, along with known positions for the signal originators being timed, define a set of intersecting hyperbola in 2D space and hyperboloids in 3D space. Three intersecting hyperboloids can be used to give a precise 3D coordinate location but only if the individual TDOA values used to define the hyperboloids are accurate.
TDOA is only useful for positioning when it can be related to the distance, or more specifically, the difference in distances, between an object of unknown position to two objects of known position. For this reason, the origin of signals from/to the two objects of known position used for TDOA measurements must be brought into time synchronization to a useful resolution to make location possible. Typically, this is done by synchronizing the objects of known position and quite often the mobile object of unknown position, to a common time reference so that the TDOA can be directly related to the difference in distances between the mentioned objects. Synchronization of these elements is not only very expensive but, especially in the case where the signals are traveling at the speed of light and being received by the mobile object of unknown position, requires that the mobile be capable of differentiating the two arrival times at a very high precision and resolution in order for the positioning information to be useful for most client aware applications.
The signal being used must also be of a consistent or predictable velocity. Sound for instance, although generally consistent in velocity at any particular moment, travels through air at significantly different rates depending on the current ambient temperature. However, this variation in velocity for sound at different ambient air temperatures can be compensated for and predictable for short periods of time.
The primary use of TDOA and the resulting conversion to Di) is for real-time location systems (RTLS). It can be in a server aware or client aware location environment or possibly both. Examples of a server-aware environment may be the centralized tracking of expensive hospital equipment to room level resolution or tracking a tool tagged to broadcast a signal giving its position. In server-aware scenarios, the client is not necessarily aware of its position. Nokia has developed a server aware positioning system recently using Bluetooth 4.0 antenna arrays and triangulation techniques for example. One downfall of an exclusive server-aware positioning environment is that there is always a limit as to the number of clients that can be tracked.
Client-aware positioning systems are systems in which the mobile client is aware of its position within the domain. A popular example of a client-aware positioning system would be a global positioning service (GPS) device made by companies for automotive guidance using the GPS satellites currently in orbit over the Earth today. The client may choose to reveal its position to a server to form a combined client/server-aware positioning system, but it would not be required to determine the client's position.
RTLS systems today are very expensive, bulky, and complicated. They typically require some type of system level clock synchronicity, which must be distributed and maintained to the mobile device. This can take a lot of time for the user and is very expensive to design-in on both the server side and the client side. In some markets, like the GPS market, this is easily offset by the scale of deployment and the number of users. Still in other markets these problems form a barrier to entry, especially in the very lucrative local/indoor client aware positioning market. Power requirements, distribution, mobile footprint, standards compliance, device HW penetration, multipath reflection, installation, and calibration are factors that drive cost of ownership and affect market adoption for the client-aware local real-time location market.
Ultra-wideband (UWB) pulse radios are widely being researched at present for their very short pulse duration and, therefore, for their ability to both avoid multipath reflection issues and, in moderation, avoid interference with common narrow, wide, and broadband frequency communications including Wi-Fi and Bluetooth. Unfortunately, most applications using UWB pulses for location and ranging, still require very high resolution timing and experience difficulty detecting the leading edge of the pulse to get an accurate time stamp.
While TDOA of synchronized signals is commonly used as a method for finding the DiD needed for multilateration to work, it is not the only method. It is possible, as the present invention will prove, to actually calculate more directly, the DiD rather than an equivalent difference in time which must then be converted to a distance. The present invention is a method for determining the DiD between an object of unknown location and two objects of known location. It requires no long term network-wide synchronization between client devices and the positioning domains infrastructure and seeks to significantly improve on the existing barriers to the market mentioned in the above background statement by simplifying the method for determining an equivalent measurement to TDOA.
Since the method of the present invention is effectively equivalent to the more common method of synchronous TDOA determination, it is often referred to as asynchronous TDOA determination in this disclosure.